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What is TCP slow start (and why do speed tests ramp up)?
In one sentence: How a new internet connection begins with a small burst of data and doubles it every round trip until something is lost or the pipe is full.
A little more
When a computer starts sending over a new connection it has no idea how much the network between it and you can carry. So TCP, the rule book most downloads and web pages use, starts with a small window of about ten packets. Every time the other end confirms they arrived, the window grows, roughly doubling every round trip. That's slow start, even though the doubling soon makes it fast.
It stops doubling when a packet is lost or the queue at the narrowest pipe starts to fill; from then on the connection grows gently and eases back a little after each loss. Because every step waits for a round trip, a far-away server (long ping) takes longer to reach full speed and longer to recover from each loss. That's one big reason speed tests pick nearby servers, use several connections at once, and sometimes leave out the first second or two.
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